Requesting Polarizability, IR, and Raman Spectra

Within the electric field linear response formalism, CASTEP enables you to calculate optical (ω = ∞) and dc (ω = 0) dielectric permittivity or optical (ω = ∞) and static (ω = 0) molecular polarizability. CASTEP calculates these properties when you specify either Efield or Phonon+Efield for the TASK keyword in the input parameters (.param) file. Specifying Efield also calculates infrared intensities (response to an electric field in the infrared range) , if the phonon k-points include a Γ-point.

The calculated results are somewhat sensitive to the value of the scissors correction. This correction amounts to the difference between the calculated and experimental band gap. If you have experimental data, you can estimate this correction by performing the band structure run with CASTEP and comparing the calculated band gap with the experimental value.

To calculate polarizability, IR and Raman spectra

  1. Choose Modules | CASTEP | Calculation from the menu bar to display the CASTEP Calculation dialog.
  2. Select the Properties tab.
  3. Select Polarizability, IR and Raman spectra in the list of properties.

  4. Choose the correct description of the system for the System type (Crystal, Molecule, or Linear molecule).

    The last two options refer to the supercell description of isolated molecules.

  5. Specify whether to Calculate Raman intensities.
  6. Click More... to open the CASTEP IR and Raman Options dialog.
  7. Optionally, alter the accuracy of the calculation by entering a new value in the Convergence tolerance field.
  8. If you know the value of the scissors operator, enter it as the Scissors operator and close the dialog.

Calculating the Polarizability, IR and Raman Spectra of an Isolated Molecule

You can use CASTEP to calculate the electric field response of isolated molecules as well as of solids. Use only the Γ-point for electronic calculations on molecular systems. In addition, the all Bands/EDFT electronic minimizer is known to be more efficient for studies of isolated molecules in supercell geometry than the density mixing minimizer. Therefore, change the Electronic minimizer to All Bands/EDFT on the SCF tab of the CASTEP Electronic Options dialog when performing such calculations.

Calculations for "molecule in a box" systems that do not require geometry optimization can be sped up if the molecular symmetry is utilized. Use the Find Symmetry tool to find and apply the symmetry of the molecule to the supercell.

It is important to select the correct description of the system as the System type when calculating the electric field response for a molecular system. The wrong system description can significantly affect the molecular polarizability values obtained.

Limitations of the Electric Field Response Implementation in CASTEP

This version of CASTEP has some limitations related to electric field response calculations. The supported settings for electric field response calculations are:

Selecting Fix occupancy on the SCF tab of the CASTEP Electronic Options dialog uses fixed orbital occupancies. This is not sufficient if DFT results from a non-spin-polarized calculation indicate that the system under investigation is nonmetallic. Attempting to study a metallic system using fixed orbital occupancies results in poor convergence of the electric field response calculation. The symptoms are very large numbers for IR and Raman intensities and for polarizability in the output file. If this is the case, investigate whether your system is metallic by running either density of states or band structure calculations.

If the configuration violates any of these restrictions, an error message displays with an explanation of the problem. You must change the offending setting to proceed with the calculation.

See Also:

Infrared spectra
Raman spectra
Requesting vibrational properties
Setting up a calculation on an isolated molecule
CASTEP Calculation dialog